Sheet gripper for printing presses



A. B. MITCHELL 2,972,300

SHEET GRIPPER FOR PRINTING PRESSES Filed April 25, 1959 Feb. 21, 1961 INVENTOR.

ARTHUR B MITCHELL ATTORNEYS 7 2,972,300 SHEET-GRIPPER FOR PRINTING PRESSES Arthur B. Mitchell, La Grange, 111., assignor to Miehleloss-Dexter, Incorporated, Chicago, [1]., a corporation of Delaware Filed Apr. 23, 1959, Ser. No. 808,469

3 Claims. (Cl. 101-412) This invention pertains to sheet gripping devices in general and has particular reference to improved sheet gripper fingers and/or pads for sheet fed printing presses or the like machines.

In a multi-color rotary type sheet fed printing press, for example, a sheet to be printed is accurately registered by suitable front and side guide devices before it is fed to the gripper members of the first impression cylinder. After having received the first color impression and during its travel through the press, the sheet is transferred between numerous sets of gripper assemblies on the various sheet carrying members until all of the respective color impressions have been printed thereon in precise superposed relation, whereupon it is trans ferred to the grippers of a delivery conveyor and carried to a delivery pile. The quality of the finished prints, therefore, is dependent to a great extent upon the ability of the successive gripper assemblies to hold and prevent slippage of the sheet under all conditions and thereby maintain its initial register position during its travel through the press.

Although the problem of controlling the sheets exists to "some extent in virtually all types of sheet "fed 'print- 'ing presses and sheet handling machines, it is most pronounced and troublesome in presses of the lithographic or Offset type wherein the impression is first printed on a resilient blanket and is then offset from the blanket onto a sheet. This is due, at least in part, to the resilient nature of the blanket on the blanket cylinder which tends to conform to the surface of the impression cylinder along the line of impression as the two cylinders roll together during operation of the press. Accordingly when a sheet on the impression cylinder passes the line of impression, the blanket squeezes all of the air from between the respective surfaces and the entire area of the sheet is thus pressed into intimate contact with the surface of the blanket. This condition, coupled with the fact that the ink used for this type of printing is relatively tacky, causes the sheet to cling rather tenaciously to the blanket cylinder. Consequently, as the sheet moves progressively past the line of impression and its leading edge is transferred to the adjacent transfer drum, the tendency of the trailing portion of the sheet to adhere to the blanket cylinder substantially increases the pull on the gripper edge of the sheet while it is held by the impression cylinder grippers and subsequently the transfer drum grippers and until the trailing portion of the sheet is completely stripped off the blanket. Unless the impression cylinder and transfer drum gripper members can effectively hold the sheet during this critical period, it will slip relatively thereto and the subsequent impressions will be printed out of register.

The slippage condition is further aggravated by the type of stock being printed and thus hard coated papers such as enameled or machine finished stock, which have a relatively low co-efiicient of friction, will slip much more readily than a soft, uncoated paper.

nited States Patent '2 in an endeavor to improve the holding power "of the grippers, namely: (1) by increasing the pressure b'etween the gripping surfaces, or (2) by increasing the coefficient of friction-of the mating or co-acting gripping surfaces.

The first approach, that is, increasing the pressure between the co-acting surfaces, has proved to be unsatisfactory for two reasons. Before the pressure can be increased to an extent sufiicient to preclude slippage under all conditions, it is necessary to increase the size "and strength of the various elements so they will withstand the heavier loads imposed thereon over an extended period of operation and this adds substantially 'to the manufacturing costs. A more serious objection, however, resides in the fact that the high pressure exerted between the fingers and pads causes the leading or gripper edge of some stocks to be crushed at the areas where ice:

the sheets are engaged by the gripper elements making not any of the modifications heretofore proposed have proved to be completely effective. It is known, for example, to provide the sheet engaging surfaces of the gripper fingers and/or pads with knurled or serrated finishes and such gripper members have proved to be more effective, atleast initially, than conventional, smooth fingers and pads. However, their efiectivenessis substantially reduced after a relatively short period of operation'due to the abrasive action of thepaper which soon wears away the projections leaving them relatively blunt.

This action is further accelerated by the continuous impact of the fingers against the pads, particularly during intermittent periods when no sheets are being fed, and

the tendency for lint and/or paper dust to pack between the surface asperities until the surfaces of the fingers and pads become substantially smooth. Consequently, in order to maintain a press at peak efficiency it becomes necessary to change the gripper elements at rather fre- 'quent intervals. This, however, is a time consuming procedure, particularly on large multi-color presses, which reduces the output of the press and thereby adds to pro- "duction costs.

' With thepresent invention I have succeeded in overcoming the disadvantages of the known methods and have provided a gripper assembly which: is completely effective in eliminating relative movement of the sheet; can be operated with a minimum pressure between the gripper fingers and pads; and which will retain and usually increase its effectiveness over a greatly extended period of operation.

In accordance with a preferred embodiment of this invention the sheet engaging surface of the gripper finger and/or the co-acting gripper pad is coated with a mu]- tiplicity of irregularly shaped, sharp edged or pointed tungsten carbide grits which are embedded in a matrix of suitable bonding material, such as by brazing, so as to become,in effect, an integral part of said surface. As

an alternative, the actual hardened surface of the gripper members may be etched chemically to present a great number of closely spaced, minute, hardened projections which also have the ability to effect a substantial mechanical interlocking relationship with the sheets when the gripper fingers are closed.

For a better understanding of the invention a preferred embodiment thereof is illustrated in the accompanying drawings wherein:

1 Figure 1--is a view-showing a gripper finger andlpad of a conventional sheet transfer drum in sheet gripping position;

Figure 2 is a view of a portion of Figure 1, at a greatly enlarged scale, showing the interlocking effect of the gripper elements with a sheet;

Figure 3 is a greatly enlarged view showing the manner in which the grits are secured to the respective surfaces; and

Figure 4 is a similar view illustrating a gripper surface as formed by chemical etching.

In Figure 1 of the drawings I have illustrated, merely by way of example, a portion of a conventional sheet transfer drum such as is commonly used singly or in sets in multi-color, unit type offset and typographic printing presses to transfer sheets from the impression cylinder of one printing unit to the impression cylinder of the next succeeding unit. Such drums are provided with a conventional gap 11 in their periphery to accommodate a gripper shaft 12 which is rotatably mounted therein and provides support for a series of gripper elements 13. The gripper elements are arranged in spaced relation along the shaft 12 and each one is provided with a gripper finger 14 which is adapted to co-act with a corresponding series of gripper pads 15 along the edge of the gap to receive and thereafter hold a sheet 16 securely in position until it is transferred to a next succeeding transfer drum or cylinder.

While I have herein illustrated the gripper elements as embodied in a sheet transfer drum because the sheet control problem is most acute in connection therewith, it is to be understood that they may be applied to any sheet carrying member such as an impression cylinder or the like to obtain improved sheet control.

As stated hereinbefore, it is imperative that the grip per elements maintain the sheet securely against any relative motion while it is under the control thereof or the succeeding impressions will be printed out of register. This represents a serious problem, particularly in offset presses, wherein the printed sheets tend to adhere to the blanket cylinder and an exceptionally strong force is required to peel or strip the sheets away from the blanket.

I have discovered that this problem can be completely eliminated and the sheets can be maintained in precise register by providing the sheet engaging surface 17 of the pad 15 and/or the co-acting surface 18 of the finger 14 with a coating of carbide grit particles which are bonded to the respective surfaces in a manner that they become, in effect, an integral part of the finger or pad.

As illustrated in Figure 2, the sharp edged particles 19 are bonded to the gripper finger and/or pad in a manner that a portion of each particle projects above the bonding material and actually embeds itself in the top and bottom surfaces of the sheet when the-grippers are closed. Thus a substantial mechanical interlocking effect is achieved and the paper cannot be moved relative to the gripper surfaces unless the resistance is great enough to tear the fibres loose. Moreover, the sharp edges of the minute particles readily penetrate the outer surfaces of the sheet thereby increasing the co-efiicient of friction at the interfaces and thus a minimum force on the gripper fingers and the associated actuating members is required for effective holding.

In order to obtain the greatest holding effect without affecting the appearance of the printed sheets, the particles are preferably of a size such that when they are bonded to the surface of the gripper pad or finger they will project above the surface of the bonding material to an extent substantially less than the thickness of the sheets. The particles will then penetrate through only the outer surface of the sheet, as illustrated in Figure 2, which is sufficient, nevertheless, to effectively hold it in position and the minute depressions formed therein along the gripper edge will not be noticeable or damaging to the finished product.

i 1 Although the illustrations are drawn at a greatly enlarged scale it will be appreciated that the grit particles used are very small in size and preferably fall within a range of .003" to .012" of an inch. The size of the particle used, however, is not particularly critical and they can be varied in accordance with the thickness of the stock to be processed. If, for example, a press is to be used predominantly for printing relatively thick cardboard, a #60 grit, which is substantially .012" in size, can be used very effectively. On the other hand, for thin papers, such as book stock or the like, a #240 grit, which is about .003" in size, would be more suitable.

It is impractical to change the gripper elements of a printing press each time a different grade of paper is to be printed, however, and I have found that grit particles ranging in size between and #180 and preferably a grit, which is approximately .004" in size, give the best results and will effectively accommodate the widest range of stock.

In the preferred form of the invention, the grits may be dispersed over the sheet engaging surface of the gripper elements, either in a uniform pattern or in a random arrangement, and they are bonded to said surface in a matrix formed by a silver or copper brazing material and in a manner that they are embedded in the brazing material to approximately one half of the grit parf'cles size, as illustrated in Figure 3. Accordingly a #150 grit particles of approximately .004" size would be embedded in the brazing material 20 to a depth of about .002 which is suificient to rigidly hold the particle in position and yet permit it to project above the surface of the bonding matrix to an extent (approximately .002) sufficient to provide the required interlocking relationship with the paper.

The improved holding power of this type of gripper surface is apparent from the following table which indicates the force, in pounds, which must be exerted upon the paper before it can be pulled out from between the co-acting gripper finger and pad surfaces. These tests were conducted on an experimental set up and in each test the pressure of the gripper finger on the paper or its co-acting pad was set to an average pressure of substantially 64 lbs. per square inch.

From the above table it is evident that under similar conditions the various carbide grit surfaced finger and pad combinations are substantially two to five times as effective as the standard smooth gripper finger and serrated pad combination. Moreover, after 8% million cycles of operation, during a portion of which time no paper was placed between the fingers and pads, it was found that the extremely hard grits had actually indented the smooth, case hardened surface of the'coacting gripper finger and that the co-efiicient of friction had actually 'become increased rather than reduced.

In the tests described above, as well as in several test applications on presses in the field, tungsten carbide grit particles were used because of their extreme hardness, which is listed as a Knoop value of 1340 or more, their sharp pointed characteristics and their ready availability. This is not to be regarded as a specific limitation, how ever, because any carbide or similar grit particle of about the same hardness and physical characteristics and having a melting point above that of a suitable bonding material, will be satisfactory. Examples of materials exhibiting this degree of hardness are. tantalum, zirconium,

beryllium, titanium, silicon and boron carbides, as well as aluminum and zirconium boride, titanium nitride and diamond particles.

Correspondingly efiective results can also be obtained with gripper fingers and/or pads, the sheet engaging surfaces of which are etched, as by chemical milling, to provide an array of relatively sharp pointed projections or pips 21 thereon as illustrated in Figure 4 In this modification the sheet engaging surface of the gripper finger and/or pad is first coated with a photosensitive resist material which is exposed to light through a screen of the proper mesh size in much the same manner as when making a photoengraved printing plate. The unexposed, soluble portion of the coating is then washed away leaving small dots of the resist material on the gripper surface which is then subjected to the action of a suitable etchant solution to etch away the bare metal until the surface takes a form substantially as illustrated in Figure 4. Depending upon the type of screen used, the points 21 are etched to a uniform depth of from .003" to .007 and they may be varied in diameter at the top between .000" to .002".

The size of the screen used when exposing the resist material may be varied much in the same manner as the size of the grit particles, but a screen which will provide about 500 dots per square inch appears to produce the most practical surface. Moreover, a regular uniform arrangement of screen elements is not essential, and the above illustration is set forth merely as an example of one way in which projections might be produced. A random arrangement is just as effective and was actually used in connection with data given below.

It also will be understood that the surfaces of the gripper elements are hardened, either before or after the etching procedure so that they will retain their physical characteristics and high co-eiiicient of friction over an extended period of operation. Any suitable hardening process may be employed, but for maximum effective life, they should be treated so as to have a minimum Knoop hardness value of about 700.

This type of surface has proved to be very eifective in holding power even surpassing, in some instances, the co-efficient of friction of the carbide grit particles, as evidenced by the following table. However, ithas a slight disadvantage with respect to the carbide coated grippers in that it has a somewhat shorter effective life because the materials commonly used for the gripper elements cannot be hardened to a comparable value.

The following data was obtained under the same conditions as that given in the above table relative to the carbide coated grippers and with the same pressure of approximately 64 lbs. per square inch exerted between the gripper fingers and pads.

Having thus disclosed preferred embodiments of my invention, what is claimed is:

1. A gripper member for a printing press or the like machine comprising a base member, a sheet engaging surface on said member, a multiplicity of irregularly shaped grit particles distributed on said surface, said grit particles having a size of approximately .004 of an inch and being embedded to approximately one half grit particle size in a matrix of brazing material applied directly to said surface, whereby to provide the required interlocking relationship with the sheet adapted to be gripped by the gripper member, and said grit particles being selected from the group consisting of tantalum, zirconium, beryllium, titanium, tungsten, silicon and boron carbides, aluminum and zirconium boride, titanium nitride and diamond particles.

2. A gripper member for a printing press or the like machine comprising a base member, a sheet engaging surface on said member, a multiplicity of irregularly shaped grit particles distributed on said surface, said grit particles ranging in size from .003 to .012 of an inch and being embedded to approximately one half grit particle size in a matrix of brazing material applied directly to said surface, whereby to provide the required interlocking relationship with the sheet adapted to be gripped by the gripper member, and said grit particles being selected from the group consisting of tantalum, zirconium, beryllium, titanium, tungsten, silicon and boron carbides, aluminum and zirconium boride, titanium nitride and diamond particles.

3. In gripper mechanism for a printing press or the like, in combination, a pair of gripper members adapted to have coasting operation for gripping a sheet therebetween, a multiplicity of irregularly shaped grit particles distributed over the sheet engaging surface of one of said members, said grit particles ranging in size from .003 to .012 of an inch and being embedded to approximately one half grit particle size in a matrix of brazing material applied directly to said surface, said other gripper member having a smooth sheet engaging surface the hardness of which is less than that provided by the grit particles, and said grit particles being selected from the group consisting of tantalum, zirconium beryllium, titanium, tungsten, silicon and boron carbides, aluminum and zirconium boride, titanium nitride and diamond particles.

References Cited in the file of this patent UNITED STATES PATENTS 1,300,610 Henderson Apr. 15, 1919 1,948,038 Kast Feb. 20, 1934 2,005,046 Newcomb June 18, 1935 2,011,181 Melton et a1. Aug. 13, 1935 2,085,845 Binkley July 6, 1937 2,161,597 Swartz June 6, 1939 2,216,250 Nelson Oct. 1, 1940 2,562,587 Swearingen July 31, 1951 2,600,913 Olson June 17, 1952 

